Method and apparatus for hybrid photoacoustic compression machining in transparent materials using filamentation by burst ultrafast laser pulses

ABSTRACT

An apparatus, system and method for the processing of orifices in materials by laser filamentation that utilizes an optical configuration that focuses the incident laser light beam in a distributed manner along the longitudinal beam axis. This distributed focusing method enables the formation of filaments over distances, and the laser and focusing parameters are adjusted to determine the filament propagation and termination points so as to develop a single/double end stopped orifice, or a through orifice. Selected transparent substrates from a stacked or nested configuration may have orifices formed therein/therethrough without affecting the adjacent substrate. These distributed focusing methods support the formation filaments with lengths well beyond ten millimeters in borosilicate glass and similar brittle materials and semiconductors.

BACKGROUND OF THE INVENTION

The present invention relates to a non-ablative method and apparatus fordrilling stopped or through orifices beginning at any depth, or in anyone of a set of stacked wafers, plates or substrates, primarily, but notlimited to, such transparent materials as glass, sapphire, silicon suchthat the structural characteristics of the orifice and surroundingmaterial exceed that found in the prior art.

There is a huge demand for drilling multiple holes in a transparentsubstrate such as one made of glass or a polycarbonate. One applicationof a drilled substrate is for use as a filter for air monitoring,particle monitoring, cytology, chemotaxis, bioassays, and the like.These commonly require orifices a few hundred nanometers to tens ofmicrometers in diameter that remain identical to each other and have ahole to surface area ratio that remains stable when produced in volume.

Currently, the prior art material processing systems produce orifices insubstrates such as glass by diamond drilling, or laser exposuretechniques such as: ablative machining; combined laser heating andcooling; and high speed laser scribing. All of the prior art systemshave low throughput times, do not work well with many of the new exoticsubstrate materials, have problems with the opacity of multiple levelsubstrate stacks, cannot attain the close orifice spacing sought,propagate cracks in the material or leave an unacceptable surfaceroughness on the orifice sides and surface surrounding the point ofinitiation as detailed below.

In current manufacturing, the singulation, treatment of wafers or glasspanels to develop orifices typically relies on diamond cutting routingor drilling.

Laser ablative machining is an active development area for singulation,dicing, scribing, cleaving, cutting, and facet treatment, but hasdisadvantages, particularly in transparent materials, such as slowprocessing speed, generation of cracks, contamination by ablationdebris, and moderated sized kerf width. Further, thermal transportduring the laser interaction can lead to large regions of collateralthermal damage (i.e. heat affected zone). While laser ablation processescan be dramatically improved by selecting lasers with wavelengths thatare strongly absorbed by the medium (for example, deep UV excimer lasersor far-infrared CO2 laser), the above disadvantages cannot be eliminateddue to the aggressive interactions inherent in this physical ablationprocess.

Alternatively, laser ablation can also be improved at the surface oftransparent media by reducing the duration of the laser pulse. This isespecially advantageous for lasers that are transparent inside theprocessing medium. When focused onto or inside transparent materials,the high laser intensity induces nonlinear absorption effects to providea dynamic opacity that can be controlled to accurately depositappropriate laser energy into a small volume of the material as definedby the focal volume. The short duration of the pulse offers severalfurther advantages over longer duration laser pulses such as eliminatingplasma reflections and reducing collateral damage through the smallcomponent of thermal diffusion and other heat transport effects duringthe much shorter time scale of such laser pulses. Femtosecond andpicosecond laser ablation therefore offer significant benefits inmachining of both opaque and transparent materials. However, machiningof transparent materials with pulses even as short as tens to hundredsof femtosecond is also associated with the formation of rough surfacesand microcracks in the vicinity of laser-formed orifices or trench thatis especially problematic for brittle materials like Alumina glasses,doped dielectrics and optical crystals. Further, ablation debris willcontaminate the nearby sample and surrounding surfaces.

A kerf-free method of cutting or scribing glass and related materialsfor orifices relies on a combination of laser heating and cooling, forexample, with a CO2 laser and a water jet. Under appropriate conditionsof heating and cooling in close proximity, high tensile stresses aregenerated that induces cracks deep into the material, that can bepropagated in flexible curvilinear paths by simply scanning the lasercooling sources across the surface. In this way, thermal-stress inducedscribing provides a clean separation of the material without thedisadvantages of a mechanical scribe or diamond saw, and with nocomponent of laser ablation to generate debris. However, the methodrelies on stress-induced crack formation to direct the scribe and amechanical or laser means to initiate the crack formation. Shortduration laser pulses generally offer the benefit of being able topropagate efficiently inside transparent materials, and locally inducemodification inside the bulk by nonlinear absorption processes at thefocal position of a lens. However, the propagation of ultrafast laserpulses (>5 MW peak power) in transparent optical media is complicated bythe strong reshaping of the spatial and temporal profile of the laserpulse through a combined action of linear and nonlinear effects such asgroup-velocity dispersion (GVD), linear diffraction, self-phasemodulation (SPM), self-focusing, multiphoton/tunnel ionization (MPI/TI)of electrons from the valence band to the conduction band, plasmadefocusing, and self-steepening. These effects play out to varyingdegrees that depend on the laser parameters, material nonlinearproperties, and the focusing condition into the material.

There are other high speed scribing techniques for flat panel display(FPD) glasses. A 100-kHz Ti:sapphire chirped-pulse-amplified laser offrequency-doubled 780 nm, 300 fs, 100 μJ output was focused into thevicinity of the rear surface of a glass substrate to exceed the glassdamage threshold, and generate voids by optical breakdown of thematerial. The voids reach the back surface due to the high repetitionrate of the laser. The connected voids produce internal stresses anddamage as well as surface ablation that facilitate dicing by mechanicalstress or thermal shock in a direction along the laser scribe line.While this method potentially offers fast scribe speeds of 300 mm/s,there exists a finite kerf width, surface damage, facet roughness, andablation debris as the internally formed voids reach the surface.

Although laser processing has been successful in overcoming many of thelimitations associated with diamond cutting, as mentioned above, newmaterial compositions have rendered the wafers and panels incapable ofbeing laser scribed.

Henceforth, a fast, economical system for drilling through or stoppedorifices in transparent materials emanating from the top or bottomsurface, that avoids the drawbacks of existing prior art systems wouldfulfill a long felt need in the materials processing industry. This newinvention utilizes and combines known and new technologies in a uniqueand novel configuration to overcome the aforementioned problems andaccomplish this.

SUMMARY OF THE INVENTION

The general purpose of the present invention, which will be describedsubsequently in greater detail, is to provide an apparatus and methodfor producing orifices in transparent substrates, generallysemiconductor materials such as Si wafers or materials such as glass orSapphire, by using filamentation by a burst of ultrafast laser pulseswith specific adjustments of the laser parameters in conjunction with adistributed focus lens assembly that creates a plurality of differentfoci wherein the principal focal waist never resides in or on thesurface of the target; so as to create a filament in the material thatdevelops an orifice in any or each member of a stacked array of thematerial wherein the orifice has a specified depth and width at adesired point of initiation and point of termination within the desiredwafer, plate or substrate. While the present disclosure focusesprimarily on the drilling of orifices it is understood that the systemand method described herein are equally applicable to the machiningprocesses of drilling, dicing, cutting and scribing targets.

A method and apparatus for drilling stopped or through orificesbeginning at any depth, or in anyone of a set of stacked wafers, platesor substrates, primarily, but not limited to, transparent materials suchthat the structural characteristics of the orifice and surroundingmaterial exceed that found in the prior art. More particularly, to anapparatus and method of multiple orifice formation in any or each memberof a stacked array of materials by a novel method using interference ofa burst of ultrafast laser pulses wherein the laser light and focusingparameters have been adjusted to create a filament inside the materialthat can create an orifice of specified depth and width at the desiredpoint of initiation and point of termination.

A novel and unique technique to create nanometer to micrometer scaleorifices in and through transparent material such as Si wafers, glass orSapphire is disclosed. It has many of the advantages mentionedheretofore and many novel features that result in a new method ofproducing non-ablatively drilled orifices in materials which is notanticipated, rendered obvious, suggested, or even implied by any of theprior art, either alone or in any combination thereof. Specifically, itoffers the following huge advances over the prior art: smoother orificesides, minimal micro crack propagation, longer/deeper orifice creation,non tapered orifices, nonlinear absorption, orifices with a consistentinternal diameter, minimized entrance distortion and reduced collateraldamage. The subject matter of the present invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. However, both the organization and method of operation,together with further advantages and objects thereof, may best beunderstood by reference to the following description taken in connectionwith accompanying drawings wherein like reference characters refer tolike elements. Other objects, features and aspects of the presentinvention are discussed in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a prior art ablative laserdrilling arrangement wherein the principal focus occurs at the topsurface of the transparent substrate;

FIG. 2 is a perspective view of a an orifice formed by the drillingarrangement of FIG. 1;

FIG. 3 is a representative side view of a prior art ablative laserdrilling arrangement wherein the principal focus occurs below the topsurface of the transparent substrate;

FIG. 4 is a perspective view of a an orifice formed by the drillingarrangement of FIG. 3;

FIG. 5 is a representative side view of an orifice ablatively drilled asthe laser arrangement of FIG. 1 wherein the primary focus occurs at thetop surface of the transparent substrate;

FIG. 6 is a diagrammatic representation of the laser drillingarrangement of the present invention wherein the primary focus occursabove the top surface of the transparent substrate;

FIG. 7 is a perspective view of an orifice scribe in a transparentsubstrate formed by the laser drilling arrangement of the presentinvention;

FIG. 8 is a representative side view of two orifices drilled by thelaser arrangement of FIG. 6;

FIG. 9 is a diagrammatic representation of the prior art ablative laserdrilling arrangement;

FIG. 10 is a diagrammatic representation of the present invention;

FIG. 11 is a diagrammatic view of the present invention utilizing adistributed focus lens arrangement;

FIG. 12 is a diagrammatic view of the present invention utilizing adistributed focus lens arrangement;

FIG. 13 is a diagrammatic view of the present invention utilizing adistributed focus lens arrangement and the distribution of focal waistswhere the principal focus is above the target;

FIG. 14 is a diagrammatic view of the present invention utilizing adistributed focus lens arrangement and the distribution of focal waistswhere the principal focus is below the target;

FIG. 15 is a diagrammatic view of the present invention of FIG. 13wherein the orifice has been drilled;

FIG. 16 is a diagrammatic view of the present invention utilizing adistributed focus lens arrangement and the distribution of focal waistswhere the principal focus is below multiple targets; and

FIGS. 17-19 show three various configurations of the distribution oflaser energy.

DETAILED DESCRIPTION

There has thus been outlined, rather broadly, the more importantfeatures of the invention in order that the detailed description thereofthat follows may be better understood and in order that the presentcontribution to the art may be better appreciated. There are, of course,additional features of the invention that will be described hereinafterand which will form the subject matter of the claims appended hereto.

Various embodiments and aspects of the disclosure will be described withreference to details discussed below. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosure.

In this respect, before explaining at least one embodiment of theinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction and to thearrangements of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments and of being practiced and carried out in various ways.Also, it is to be understood that the phraseology and terminologyemployed herein are for the purpose of descriptions and should not beregarded as limiting.

Unless defined otherwise, all technical and scientific terms used hereinare intended to have the same meaning as commonly understood to one ofordinary skill in the art. Unless otherwise indicated, such as throughcontext, as used herein, the following terms are intended to have thefollowing meanings:

As used herein, the term ablative drilling refers to a method ofmachining a target (generally by cutting or drilling of a substrate bythe removal of material) surface by irradiating it with a laser beam. Atlow laser flux, the material is heated by the absorbed laser energy andevaporates or sublimates. At high laser flux, the material is typicallyconverted to a plasma. Usually, laser ablation refers to removingmaterial with a pulsed laser, but it is possible to ablate material witha continuous wave laser beam if the laser intensity is high enough.Ablative drilling or cutting techniques are characterized by thecreation of a debris field, the presence of a liquid/molten phase atsome point during the material removal process, and the creation of anejecta mound at the entrance and or exit of the feature.

As used herein, the term “photoacoustic drilling” refers to a method ofmachining a target (generally by cutting or drilling of a substrate froma solid by irradiating it with a lower pulse energy light beam than isused in ablative drilling or cutting techniques. Through the processesof optical absorption followed by thermoelastic expansion, broadbandacoustic waves are generated within the irradiated material to form apathway of compressed material about the beam propagation axis (commonwith the axis of the orifice) therein that is characterized by a smoothwall orifice, a minimized or eliminated ejecta and minimized micro crackformation in the material.

As used herein the term “optical efficiency” relates to the ratio of thefluence at the principal focal waist to the total incident fluence atthe clear aperture of the focusing element or assembly.

As used herein, the term “transparent” means a material that is at leastpartially transparent to an incident optical beam. More preferably, atransparent substrate is characterized by absorption depth that issufficiently large to support the generation of an internal filamentmodified array by an incident beam according to embodiments describedherein. Stated otherwise, a material having an absorption spectrum andthickness such that at least a portion of the incident beam istransmitted in the linear absorption regime.

As used herein, the term “filament modified zone” refers to a filamentregion within a substrate characterized by a region of compressiondefined by the optical beam path.

As used herein, the phrases “burst”, “burst mode”, or “burst of laserpulses” refer to a collection of laser pulses having a relative temporalspacing that is substantially smaller than the repetition period of thelaser. It is to be understood that the temporal spacing between pulseswithin a burst may be constant or variable and that the amplitude ofpulses within a burst may be variable, for example, for the purpose ofcreating optimized or pre-determined filament modified zones within thetarget material. In some embodiments, a burst of laser pulses may beformed with variations in the intensities or energies of the pulsesmaking up the burst.

As used herein, the phrase “geometric focus” refers to the normaloptical path along which light travels based on the curvature of thelens, with a beam waist located according to the simple lens equationcommon to optics. It is used to distinguish between the optical focuscreated by the position of the lenses and their relation to one anotherand the constriction events created by thermal distortion in the targetmaterials providing, in effect, a quasi-Rayleigh length on the order ofup to 15 mm, which is particularly uncommon and related to the inventivenature of this work.

As used herein, the term “substrate” means a glass or a semiconductorand may be selected from the group consisting of transparent ceramics,polymers, transparent conductors, wide bandgap glasses, crystals,crystalline quartz, diamond, sapphire, rare earth formulations, metaloxides for displays and amorphous oxides in polished or unpolishedcondition with or without coatings, and meant to cover any of thegeometric configurations thereof such as but not limited to plates andwafers. The substrate may comprise two or more layers wherein a locationof a beam focus of the focused laser beam is selected to generatefilament arrays within at least one of the two or more layers. Themultilayer substrate may comprise multi-layer flat panel display glass,such as a liquid crystal display (LCD), flat panel display (FPD), andorganic light emitting display (OLED). The substrate may also beselected from the group consisting of autoglass, tubing, windows,biochips, optical sensors, planar lightwave circuits, optical fibers,drinking glass ware, art glass, silicon, 111-V semiconductors,microelectronic chips, memory chips, sensor chips, electro-opticallenses, flat displays, handheld computing devices requiring strong covermaterials, light emitting diodes (LED), laser diodes (LD), and verticalcavity surface emitting laser (VeSEL). Targets or target materials aregenerally selected from substrates. As used herein the “principal focalwaist” refers to the most tightly focused and strongest focal intensityof the beam after final focusing (after passing through the finaloptical element assembly prior to light incidence upon the target). Itmay also be used interchangeably with the term “principal focus.” Theterm “secondary focal waist” refers to any of the other foci in thedistributed beam having a lesser intensity than the principal focalwaist. It may also be used interchangeably with the term “secondaryfocus’ or “secondary foci.”

As used herein the term “filament” refers to any light beam travelingthrough a medium wherein the Kerr effect can be observed or measured.

As used herein, “laser filamentation” is the act of creating filamentsin a material through the use of a laser. As used herein the term“sacrificial layer” refers to a material that can be removeably appliedto the target material.

As used herein the term “machining” or “modification” encompasses theprocesses of drilling orifices, cutting, scribing or dicing a surface orvolume of a target or substrate.

As used herein the term “focal distribution” refers to spatiotemporaldistribution of incident light rays passing through a lens assembly thatin its aggregate is a positive lens. Generally, herein their subsequentconvergence of spots of useful intensity as a function from the distancefrom the center of the focusing lens is discussed.

As used herein the terms “critical energy level,” “threshold energylevel” and “minimum energy level” all refer to the least amount ofenergy that must be put into or onto a target to initiate the occurrenceof a transient process in the target material such as but not limited toablative machining, photoacoustic machining, and the Kerr effect.

As used herein the term “aberrative lens” refers to a focusing lens thatis not a perfect lens wherein the lens curvature in the X plane does notequal the lens curvature in the Y plane so as to create a distributedfocal pattern with incident light that passes through the lens. Apositive aberrative lens is a focally converging lens and a negativeaberrative lens is a focally diverging lens.

As used herein, the terms, “comprises” and “comprising” are to beconstrued as being inclusive and open ended, and not exclusive.Specifically, when used in the specification and claims, the terms,“comprises” and “comprising” and variations thereof mean the specifiedfeatures, steps or components are included. These terms are not to beinterpreted to exclude the presence of other features, steps orcomponents.

As used herein, the term “exemplary” means “serving as an example,instance, or illustration,” and should not be construed as preferred oradvantageous over other configurations disclosed herein.

As used herein, the terms “about” and “approximately” are meant to covervariations that may exist in the upper and lower limits of the ranges ofvalues, such as variations in properties, parameters, and dimensions. Inone non-limiting example, the terms “about” and “approximately” meanplus or minus 10 percent or less.

The main objective of the present invention is to provide fast, reliableand economical non-ablative laser machining technique to initiateorifices (stopped/blind or through orifices) in the target material thatmay be initiated below or above a single of multiple stacked targetmaterial by filamentation by a burst(s) of ultrafast laser pulses. Ultrashort lasers offer high intensity to micromachine, to modify and toprocess surfaces cleanly by aggressively driving multi-photon, tunnelionization, and electron-avalanche processes. The issue at hand is howto put enough energy in the transparent material of the target, lessthan that used in ablative drilling, but beyond the critical energylevel to initiate and maintain photoacoustic compression so as to createa filament that modifies the index of refraction at the focal points inthe material and does not encounter optical breakdown (as encountered bythe prior art ablative drilling systems) such that continued refocusingof the laser beam in the target material can continue over longdistances, enough so that even multiple stacked substrates can bedrilled simultaneously with negligible taper over the drilled distance,a relatively smooth orifice wall and can initiate from above, below orwithin the target material.

Generally, in the prior art, laser ablation techniques that utilize ahigh energy pulsed laser beam that is focused to a single principalfocus above, within or at a surface of the material, have been used tomachine transparent materials. As shown in FIG. 1 the incident laserlight beam 2 passes through a focusing assembly passing through a finalfocusing lens 4 so as to focus a non-distributed light beam 6 that has afocal waist 8 at the surface of the target 10. As can be seen in FIG. 3,optionally, the non-distributed light beam may be focused such that thefocal waist resides within the target. Generally these techniques use aperfect spherical focusing lens 12, that is to say a lens that isnon-aberrated that has curvature in the X plane that equals thecurvature in the Y plane (Cx=Cy) or alternatively with a focusingelement assembly that produces a non distributed beam having a singlefocus 14 as shown in FIG. 9. This creates a tight beam spot that is thendelivered on (FIG. 1) or in the target substrate material 10. (FIG. 3)FIG. 2 illustrates the geometry of a machined slot 16 cut with thetechnique of FIG. 1, and FIG. 4 illustrates the geometry of an oblongorifice 18 made with the technique of FIG. 3.

Propagation of intense ultrafast laser pulses in different optical mediahas been well studied. Nonlinear refractive index of a material is afunction of laser intensity. Having an intense laser pulse with Gaussianprofile, wherein the central part of the pulse has much higher intensitythan the tails, means the refractive index varies for the central andsurrounding areas of the material seeing the laser beam pulse. As aresult, during propagation of such laser pulse, the pulse collapsesautomatically. This nonlinear phenomenon is known in the industry asself-focusing. Self-focusing can be promoted also using a lens in thebeam path. In the focal region the laser beam intensity reaches a valuethat is sufficient to cause multiple-ionization, tunnel ionization andavalanche ionization, which creates plasma in the material. Plasmacauses the laser beam to defocus and refocus back to form the nextplasma volume. The inherent problem with a single focus in anon-distributed beam is that the process ends after the laser pulseslose all their energy and are unable to refocus as discussed below.

This ablative method develops a filament in the material 10 of a lengthof up to 30 microns until it exceeds the optical breakdown threshold forthat material and optical breakdown (OB) 16 occurs. (FIG. 9) At OB themaximum threshold fluence (the energy delivered per unit area, in unitsof J/m2) is reached and the orifice diameter narrows and ablativemachining or drilling ceases to proceed any deeper. This is the obviousdrawback to using the prior art methods as they limit the size of theorifice that can be drilled, cause a rough orifice wall and result in anorifice with a taper 22 having a different diameter at the top andbottom surfaces of the target 10. (FIG. 5) This occurs because inablative machining, the beam has central focus 8 (also referred to as aprincipal focal waist) at the surface of the target 10 causing localizedheating and thermal expansion therein heating the surface of thematerial 10 to its boiling point and generating a keyhole. The keyholeleads to a sudden increase in optical absorptivity quickly deepening theorifice. As the orifice deepens and the material boils, vapor generatederodes the molten walls blowing ejecta 20 out and further enlarging theorifice 22. As this occurs, the ablated material applies a pulse of highpressure to the surface underneath it as it expands. The effect issimilar to hitting the surface with a hammer and brittle materials areeasily cracked. Additionally, brittle materials are particularlysensitive to thermal fracture which is a feature exploited in thermalstress cracking but not desired in orifice drilling. OB generally isreached when the debris is not ejected, a bubble is created in theorifice 22 or there is a violent ablation that cracks the target in thearea of the orifice 22. Anyone or combination of these effects causesthe beam 6 to scatter from this point or be fully absorbed not leavingenough beam power (fluence) to drill down through the material 10 anyfurther. Additionally, this creates a distortion or roughness known asthe ablative ejecta mound 20 found around the initiating point at thesurface of the target substrate 10. (FIG. 5)

Another problem with laser ablation techniques is that the orifices itdrills are not of a uniform diameter as the laser beam filamentationchanges its diameter as a function of distance. This is described as theRayleigh range and is the distance along the propagation direction of abeam from the focal waist to the place where the area of the crosssection is doubled. This results in a tapered orifice 22 as shown inFIGS. 2 and 5.

The present invention solves the optical breakdown problem, minimizesthe orifice roughness and the ablative ejecta mound, and eliminates thetapering diameter 5 orifice.

The present disclosure provides devices, systems and methods for theprocessing of orifices in transparent materials by laser inducedphotoacoustic compression. Unlike previously known methods of lasermaterial machining, embodiments of the present invention utilize anoptical configuration that focuses the incident beam 2 in a distributedmanner along the longitudinal beam axis such that there is a linearalignment of the principal focus 8 and secondary foci 24 (coincident tothe linear axis of the orifice but vertically displaced from theprincipal focus or focal waist) to allow the continual refocusing of theincident beam 2 as it travels through the material 10 thereby enablingthe creation of a filament that modifies the index of refraction alongthe beam path in the material 10 and does not encounter opticalbreakdown (as seen in the prior art ablative drilling systems both withand without the use of rudimentary filamentation) such that continuedrefocusing of the laser beam in the target material can continue overlong distances. (FIG. 6)

This distributed focusing method allows for the “dumping” or reductionof unnecessary energy from the incident beam 2 found at the principalfocal waist 8 by the creation of secondary foci 24 by the distributedfocusing elements assembly 26, and by positioning the location of theprincipal focal waist 8 from on or in the material, to outside thematerial 10. This dumping of beam fluence combined with the linearalignment of the principal focal waist 8 and secondary focal waists 24,enables the formation of filaments over distances well beyond thoseachieved to date using previously known methods (and well beyond 1 mm)while maintaining a sufficient laser intensity (fluence μJ/cm2) toaccomplish actual modification and compression over the entire length ofthe filament zone. This distributed focusing method supports theformation of filaments with lengths well beyond one millimeter and yetmaintaining an energy density beneath the optical breakdown threshold ofthe material with intensity enough so that even multiple stackedsubstrates can be drilled simultaneously across dissimilar materials(such as air or polymer gaps between layers of target material) withnegligible taper over the drilled distance, (FIG. 7) and a relativelysmooth walled orifice wall that can be initiated from above, below orwithin the target material. Propagating a non-tapered wall slit 23 in atarget 10 is accomplished by the relative movement of the target 10while machining an orifice.

The optical density of the laser pulse initiates a self focusingphenomena and generates a filament of sufficient intensity tonon-ablative initial photoacoustic compression in a zonewithin/about/around the filament so as to create a linear symmetricalvoid of substantially constant diameter coincident with the filament,and also causes successive self focusing and defocusing of said laserpulse that coupled with the energy input by the secondary focal waistsof the distributed beam forms a filament that directs/guides theformation of the orifice across or through the specified regions of thetarget material. The resultant orifice can be formed without removal ofmaterial from the target, but rather by a photoacoustic compression ofthe target material about the periphery of the orifice formed.

It is known that the fluence levels at the surface of the target 10 area function of the incident beam intensity and the specific distributedfocusing elements assembly, and are adjusted for the specificmaterial(s), target(s) thickness, desired speed of machining, totalorifice depth and orifice diameter. Additionally, the depth of theorifice drilled is dependent on the depth over which the laser energy isabsorbed, and thus the amount of material removed by a single laserpulse, depends on the material's optical properties and the laserwavelength and pulse length. For this reason a wide range of processparameters are listed herein with each particular substrate and matchingapplication requiring empirical determination for the optimal resultswith the system and materials used. As such, the entry point on thetarget 10 may undergo some minimal ablative ejecta mound formation 20 ifthe fluence levels at the surface are high enough to initiate momentary,localized ablative (vaporized) machining, although this plasma creationis not necessary. In certain circumstances it may be desirable toutilize a fluence level at the target surface that is intense enough tocreate the transient, momentary ablative drilling to give a broadbevelled entry yet have the remainder of the orifice 22 of uniformdiameter FIG. 8 as would be created by a distributed focus hybriddrilling method using an energy level that permits a momentary ablativetechnique followed by a continual photoacoustic compression technique.This can be accomplished by the present invention by selection of afluence level at the target surface that balances the linear absorptionagainst the non linear absorption of the beam in the material such thatthe fluence level required for ablative machining will be exhausted atthe desired depth of the bevelled (or other geometric configuration).This hybrid technique will result in a minor ejecta mound 20 that can beeliminated if a sacrificial layer 30 is applied to the target surface.Common sacrificial layers are resins or polymers such as but not limitedto PVA, Methacrylate or PEG, and generally need only be in the range of1 to 300 microns thick (although the 10-30 micron range would beutilized for transparent material machining) and are commonly applied byspraying the sacrificial layer onto the target material. The sacrificiallayer will inhibit the formation of an ejecta mound on the target 10 bypreventing molten debris from attaching itself to the surface, attachinginstead to the removable sacrificial material as is well known in theart.

To accomplish photoacoustic compression machining requires the followingsystem:

-   -   A burst pulse laser system capable of generating a beam        comprising a programmable train of pulses containing from 2 to        50 subpulses within the burst pulse envelope. Further the laser        system needs to be able to generate average power from 1 to 200        watts depending on the target material utilized, typically this        range would be in the range of 50 to 100 watts for borosilicate        glass.    -   A distributed focusing element assembly (potentially comprising        positive and negative lenses but having a positive focusing        effect in the aggregate) capable of producing a weakly        convergent, multi foci spatial beam profile where the incident        fluence at the target material is sufficient to cause        Kerr-effect self-focusing and propagation.    -   An optical delivery system capable of delivering the beam to the        target.

Commercial operation would also require translational capability of thematerial (or beam) relative to the optics (or vice versa) orcoordinated/compound motion driven by a system control computer.

The use of this system to drill photoacoustic compression orificesrequires the following conditions be manipulated for the specifictarget(s): the properties of the distributed focus element assembly; theburst pulsed laser beam characteristics; and the location of theprincipal focus.

The distributed focus element assembly 26 may be of a plethora ofgenerally known focusing elements commonly employed in the art such asaspheric plates, telecentric lenses, non-telecentric lenses, asphericlenses, annularly faceted lenses, custom ground aberrated (non-perfect)lenses, a combination of positive and negative lenses or a series ofcorrective plates (phase shift masking), any optical element tilted withrespect to the incident beam, and actively compensated optical elementscapable of manipulating beam propagation. The principal focal waist of acandidate optical element assembly as discussed above, generally willnot contain more than 90% nor less than 50% of incident beam fluence atthe principal focal waist. Although in specific instances the opticalefficiency of the distributed focus element assembly 26 may approach99%. FIG. 10 illustrates a non-aspherical, aberrated lens 34 as would beused in the aforementioned process. The actual optical efficiency of thedistributed focus element assembly 26 will have to be fine-tuned foreach specific application. The users will create a set of empiricaltables tailored for each transparent material, the physicalconfiguration and characteristics of the target as well as the specificlaser parameters. Silicon Carbide, Gallium Phosphide, sapphire,strengthened glass etc., each has its own values. This table isexperimentally determined by creating a filament within the material(adjusting the parameters of laser power, repetition rate, focusposition and lens characteristics as described above) and ensuring thatthere is sufficient fluence to induce a plane of cleavage or axis ofphotoacoustic compression to create an orifice. A sample opticalefficiency for drilling a 5 micron diameter through orifice (asillustrated in FIG. 11) in a 2 mm thick single, planar target made ofborosilicate with a 1 micron wavelength, 50 watt laser outputting aburst pulse of 100 energy having a frequency (repetition rate) thatwould lie in the 1 MHz range is 65% wherein the principal focal waist ofthe beam resides 1 mm off of the desired point of initiation.

It is to be noted that there is also a set of physical parameters thatmust be met by this photoacoustic compression drilling process. Lookingat FIGS. 11 and 12 it can be seen that the beam spot diameter 38>thefilament diameter 40>the orifice diameter 42. Additionally thedistributed beam's primary focal waist 8 is never in or on the surfaceof the target material 10 into which a filament is created.

The location of the principal focal waist 8 is generally in the range of500 μm to 300 mm off of the desired point of initiation. This is knownas the energy dump distance 32. It also is determined by the creation anempirical table tailored for each transparent material, the physicalconfiguration and characteristics of the target as well as the laserparameters. It is extrapolated from the table created by the methodnoted above.

The laser beam energy properties are as follows: a pulse energy in thebeam between 0.5 μJ to 1000 μJ the repetition rate from 1 Hz to 2 MHz(the repetition rate defines the speed of sample movement and thespacing between neighboring filaments). The diameter and length of thefilament may be adjusted by changing the temporal energy distributionpresent within each burst envelope. FIGS. 17-19 illustrate examples ofthree different temporal energy distributions of a burst pulsed lasersignal. The rising and falling burst envelope profiles of FIG. 19represent a particularly useful means of process control especially welladapted for removing thin metal layers from dielectric materials.

Looking at FIGS. 13-16 collectively, the mechanism of the presentinvention can best be illustrated. Herein, burst picosecond pulsed lightis used because the total amount of energy deposited in the targetmaterial is low and photoacoustic compression can proceed withoutcracking the material, and less heat is generated in the target materialthus efficient smaller packets of energy are deposited in the materialso that the material can be raised incrementally from the ground stateto a maximally excited state without compromising the integrity of thematerial in the vicinity of the filament.

The actual physical process occurs as described herein. The principalfocal waist of the incident light beam of the pulsed burst laser isdelivered via a distributed focusing element assembly to a point inspace above or below (but never within) the target material in which thefilament is to be created. This will create on the target surface a spotas well as white light generation. The spot diameter on the targetsurface will exceed the filament diameter and the desired feature(orifice, slot, etc.) diameter. The amount of energy thus incident inthe spot on surface being greater than the critical energy for producingthe quadratic electro-optic effect (Kerr effect—where the change in therefractive index of the material is proportional to the applied electricfield) but is lower than the critical energy required to induce ablativeprocesses and more explicitly below the optical breakdown threshold ofthe material. Self-focusing occurs above a critical power that satisfiesthe relationship whereby the power is inversely related to the productof the real and complex indices of refraction for the target material.Photoacoustic compression proceeds as a consequence of maintaining therequired power in the target material over time scales such thatbalancing between the self-focus condition and the optical breakdowncondition can be maintained. This photoacoustic compression is theresult of a uniform and high power filament formation and propagationprocess whereby material is rearranged in favor of removal via ablativeprocesses. The extraordinarily long filament thus produced is fomentedby the presence of spatially extended secondary foci created by thedistributed focusing element assembly, maintaining the self focusingeffect without reaching optical breakdown. In this assembly, a largenumber of marginal and paraxial rays converge at different spatiallocations relative to the principal focus. These secondary foci existand extend into infinite space but are only of useful intensity over alimited range that empirically corresponds to the thickness of thetarget. By focusing the energy of the second foci at a lower level belowthe substrate surface but at the active bottom face of the filamentevent allows the laser energy access to the bulk of the material whileavoiding absorption by plasma and scattering by debris.

The distributed focal element assembly can be a single aberrated focallens placed in the path of the incident laser beam to develop whatappears to be an unevenly distributed focus of the incident beam into adistributed focus beam path containing a principal focal waist and aseries of linearly arranged secondary focal waists (foci). The alignmentof these foci is collinear with the linear axis of the orifice 42. Notethat the principal focal waist 8 is never on or in the target material10. In FIG. 13 the principal focal waist is above the target materialand in FIG. 14 it is below the target material 10 as the orifice 42 maybe initiated above or below the principal focal waist 8 by virtue of thesymmetric and non-linear properties of the focused beam. Thus a beamspot 52 (approximately 10 μm distance) resides on the surface of thetarget 10 and the weaker secondary focal waists collinearly residewithin the target because the material acts as the final optical elementcreating these focal points as the electric field of the laser altersthe indices of refraction of the target. This distributed focus allowsthe amount of laser energy to be deposited in the material so as to forma filament line or zone 60. With multiple linear aligned foci and byallowing the material to act as the final lens, the target material whenbombarded with ultrafast burst pulse laser beams, undergoes numerous,successive, localized heatings which thermally induced changes in thematerial's local refractive index along the path of the liner alignedfoci causing a lengthy untapered filament 60 to be developed in thetarget followed by an acoustic compression wave that annularlycompresses the material in the desired region creating a void and a ringof compressed material about the filamentation path. Then the beamrefocuses and the refocused beam combined with the energy at thesecondary focal waists maintains the critical energy level and thischain of events repeats itself so as to drill an orifice capable of1500:1 aspect ratio (length of orifice/diameter of orifice) with littleto no taper and an entrance orifice size and exit orifice size that areeffectively the same diameter. This is unlike the prior art that focusesthe energy on the top surface of or within the target material resultingin a short filamentation distance until the optical breakdown is reachedand filamentation degrades or ceases.

FIG. 16 illustrates the drilling of orifices in the bottom two of threeplanar targets 10 in a stacked configuration with an air gap betweenthem wherein the principal focal waist 8 is positioned below the finaltarget 10. The hole can be drilled from the top or the bottom or themiddle of a multiple layer setup, but the drilling event always occursthe same distance from the principal focal waist if the same lens setand curvature is used. The focal waist is always outside of the materialand never reaches the substrate surface.

The method of drilling orifices is through photoacoustic compression isaccomplished by the following sequence of steps:

1. passing laser energy pulses from a laser source through a selecteddistributive-focus lens focusing assembly;

2. adjusting the relative distance and or angle of saiddistributive-focus lens focusing assembly in relation to a laser sourceso as to focus the laser energy pulses in a distributed focusconfiguration to create a principal focal waist and at least onesecondary focal waist;

3. adjusting the principal focal waist or the target such that theprincipal focal waist will not reside on or in the target that is beingmachined;

4. adjusting the focus such that the spot of laser fluence on thesurface of the target that is located below or above said principalfocal waist, has a diameter that is always larger than a diameter of afilamentation that is formed in the target;

5. adjusting the fluence level of the secondary focal waists are ofsufficient intensity and number to ensure propagation of a photoacousticcompressive machining through the desired volume of the target; and

6. applying at least one burst of laser pulses of a suitable wavelength,suitable burst pulse repetition rate and suitable burst pulse energyfrom the laser source to the target through the selecteddistributive-focus lens focusing assembly, wherein the total amount ofpulse energy or fluence, applied to the target at a spot where the laserpulse contacts the point of initiation of machining on the target, isgreater that the critical energy level required to initiate andpropagate photoacoustic compression machining, yet is lower than thethreshold critical energy level required to initiate ablative machining;and

7. stopping the burst of laser pulses when the desired machining hasbeen completed.

As mentioned earlier, there may be specific orifice configurationswherein a tapered entrance to the orifice may be desired. This isaccomplished by initiation of the orifice with a laser fluence levelthat is capable of ablative machining for a desired distance andcompleting the drilling with a laser fluence level below the criticallevel for ablative machining yet above the critical level forphotoacoustic machining to the desired depth in that material. This typeof orifice formation may also utilize the application of a removablesacrificial layer on the surface of the target. This would allow theformation of the ejecta mound onto the sacrificial layer such that theejecta mound could be removed along with the sacrificial layer at alater time. Such an orifice drilled by a hybrid ablative andphotoacoustic compression method of machining would be performed throughthe following steps, although the application of the sacrificial layerneed be utilized and if utilized need not occur first:

1. applying a sacrificial layer to at least one surface of a target;

2. passing laser energy pulses from a laser source through a selecteddistributive-focus lens focusing assembly;

3. adjusting the relative distance and or angle of saiddistributive-focus lens focusing assembly in relation to a laser sourceso as to focus the laser energy pulses in a distributed focusconfiguration to create a principal focal waist and at least onesecondary focal waist;

4. adjusting the principal focal waist or the target such that theprincipal focal waist will not reside on or in the target that is beingmachined;

5. adjusting the focus such that the spot of laser fluence on thesurface of the target that is located below or above said principalfocal waist;

6. adjusting the spot of laser fluence on the surface of the target suchthat it has a diameter that is always larger than a diameter of afilamentation that is to be formed in the target;

7. ensuring the fluence level of the secondary focal waists are ofsufficient intensity and number to ensure propagation of a photoacousticcompressive machining through the desired volume of the target; and

8. applying at least one burst of laser pulses of a suitable wavelength,suitable burst pulse repetition rate and suitable burst pulse energyfrom the laser source to the target through the selecteddistributive-focus lens focusing assembly, wherein the total amount ofpulse energy or fluence, applied to the target at a spot where the laserpulse contacts the point of initiation of machining on the target, isgreater than the critical energy level required to initiate ablativemachining to the desired depth and thereinafter the fluence energy atthe bottom of the ablatively drilled orifice is greater than thecritical energy level to initiate and propagate a filamentation andphotoacoustic compression machining, yet is lower than the thresholdcritical energy level required to initiate ablative machining; and

9. stopping the burst of laser pulses and filamentation when the desiredmachining has been completed.

The various parameters of the laser properties, the location of theprincipal focal waist, and the final focusing lens arrangements as wellas the characteristics of the orifice created are set forth in thefollowing table. It is to be noted that they are represented in rangesas their values vary greatly with the type of the target material, itsthickness and the size and location of the desired orifice. Thefollowing chart details the ranges of the various system variables usedto accomplish the drilling of uniform orifices in any of a plethora oftransparent materials.

Laser Properties Wavelength 5 microns or less Pulse width 10 nanosecondsor less Freq (laser pulse repetition rate) 1 Hz to 2 MegaHz Averagepower 200-1 watt Number of sub pulses per burst 1 to 50 Sub pulsespacing 0.1 femtosecond to 10 microsecond Pulse energy .5 micro Joules(μJ) to 10 micro Joules (μJ) (Average power/repetition rate) watts/l/secOrifice Properties Min Orifice Diameter .5 microns Max Orifice Diameter5 microns Max Orifice Depth 10 mm in borosilicate glass Typical AspectRatio 1500:1 Max Aspect Ratio 2500:1 Aberrated lens ratio where theCx:Cy ratio of the lenses are (−5 to 4,000) Orifice Sidewall Smoothness<5 micron ave. roughness (Material Independent) (e.g., Si, SiC, SiN,GaAs, GaN, InGaP) Orifice Side Wall Taper Negligible across 10,000micron depth (Material Independent) Beam Properties Focal Distribution−5 to 4,000 M² 1.00-5.00

As noted earlier the parameters above vary with the target. In the wayof an operational exemplary, to drill a 3 micron hole 2 mm deep in atransparent substrate the following apparatus and parameters would beused: a 1064 nanometer wavelength laser; 65 watts of average power; 10μJ pulse energy; 15 subpulses per burst; and a 1 MHz repetition rate.This would be focused with an aberated lens delivering distributed fociover 2 mm of space (filament active zone is 2 mm long) focusing 0.5microns to 100 mm above the top surface depending upon the material.

It is to be understood that the invention is not limited in itsapplication to the arrangements of the components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced and carried out withvarious different ordered steps. Also, it is to be understood that thephraseology and terminology employed herein are for the purpose ofdescriptions and should not be regarded as limiting. As such, thoseskilled in the art will appreciate that the conception, upon which thisdisclosure is based, may readily be utilized as a basis for thedesigning of other structures, methods and systems for carrying out theseveral purposes of the present invention. It is important, therefore,that the claims be regarded as including such equivalent constructionsinsofar as they do not depart from the spirit and scope of the presentinvention.

I claim:
 1. A method of laser processing a target, comprising the steps of: providing a beam having bursts of laser pulses, the target being made of a material that is transparent to the laser beam, each burst having between 2 and 50 laser pulses, each laser pulse having a pulse width of less than 10 nanoseconds; passing the laser beam through an aberrated focusing lens; focusing the laser beam using the aberrated focusing lens, the laser beam focused in a distributed manner along a longitudinal beam axis, the aberrations in the aberrated focusing lens developing the distributed focus; positioning the distributed focus with respect to the target; delivering the focused laser beam to the target, the focused laser beam having sufficient fluence to initiate Kerr effect self-focusing of the laser beam, thereby generating a laser filament that propagates along the longitudinal beam axis; and adjusting the pulse energy and distributed focusing to guide and maintain the laser filament between a desired point of initiation and a desired point of termination in the target, the laser filament creating a void coincident with the laser filament by annularly compressing the transparent material about the longitudinal beam axis.
 2. The laser processing method of claim 1, wherein the distributed focus has a plurality of foci, including a principal focus, the principal focus located below the target.
 3. The laser processing method of claim 1, wherein the laser beam has a burst repetition rate in the range of 1 hertz to 2 megahertz and a temporal spacing between the laser pulses within a burst is in the range of 0.1 femtosecond to 10 microseconds.
 4. The laser processing method of claim 1, wherein the transparent material is one of a ceramic, a glass, a crystal, and a semiconductor.
 5. The laser processing method of claim 1, wherein the laser beam has a wavelength less than 5 micrometers.
 6. The laser processing method of claim 1, wherein each laser pulse has an energy in the range of 0.5 microjoules to 1000 microjoules.
 7. The laser processing method of claim 1, wherein the filament has a length longer than 1 millimeter in the target.
 8. The laser processing method of claim 1, wherein the focusing lens is an aspheric lens.
 9. The laser processing method of claim 1, wherein the focusing lens is a focusing assembly that includes an aspheric plate.
 10. The laser processing method of claim 1, wherein the focusing lens includes an optical element tilted with respect to the laser beam.
 11. The laser processing method of claim 1, wherein the target includes two or more layers and the adjusting of the distributed focus selectively generates the laser filament within at least one of the layers.
 12. The laser processing method of claim 1, wherein the void has an average sidewall roughness of less than 5 micrometers.
 13. The laser processing method of claim 1, wherein the void is linearly symmetric, having substantially constant diameter along the length of the void.
 14. The laser processing method of claim 1, wherein the void is created in the target without removing any of the transparent material from the target.
 15. The laser processing method of claim 1, wherein the void extends to a surface of the target to form an orifice.
 16. The laser processing method of claim 15, wherein a tapered entrance to the orifice is made by adjusting the pulse energy and distributed focusing to ablate material to a desired depth in the material.
 17. The laser processing method of claim 15, wherein a sacrificial layer is applied to the surface of the target prior to making the orifice and removed from the surface after making the orifice.
 18. The laser processing method of claim 1, including the additional step of translating the target laterally relative to the longitudinal beam axis, whereby a plurality of holes are drilled into the target by repeating the delivering and translating steps. 